Avalanche Forecast
Regions: Cascades - East.
Recent storm snow from the past week continues to settle and strengthen. Watch for areas of wind transported snow as winds increase and change direction Saturday. Due to the uncertainty that still exists around the distribution and sensitivity of buried persistent weak layers, choose terrain to avoid the lower likelihood/high consequence of a large persistent slab avalanche.
Detailed Forecast
Cold and fair weather Saturday will continue to allow storm snow weakness to gain strength. Watch for areas of newly formed wind slabs in exposed terrain especially near and above treeline. Winds are expected to increase and shift ahead of the approaching system Sunday causing wind slabs to form on a variety of aspects. Take the time to identify areas of wind loaded snow near ridgelines and mid-slope cross loaded features.Â
Lots of uncertainty exists around the distribution and sensitivity of the 12/15 layer. Limited observations on this layer make it difficult to forecast. When uncertainty goes up, terrain selection can go down to maintain wide margins of safety. Time and patience will allow this layer to gain strength.Â
Snowpack Discussion
A few inches of fresh snow fell during  the day Friday adding to the snow totals for the past week. One to three feet of recent snow now sits atop the 12/15 crust/persistent weak layer. Weaknesses within the storm snow observed over the past week are gaining strength leading to an overall stabilizing trend.
Ridge top winds Wednesday and Thursday redistributed snow in exposed and wind prone areas forming wind slabs on a variety of aspects. Limited information about the location and sensitivity of these winds slabs has been received.Â
A large amount of uncertainty exists around the 12/15 crust/persistent weak layer. This layer has generally been observed at elevations below 6000 feet. Snowpack test in a few locations continue to demonstrate results and the potential for an avalanche to propagate. The limited amount of incoming information makes it difficult to paint a clear picture of this potential avalanche problem.Â
Snowdepth decreases substantially the further east from the Cascade crest. In many areas below treeline, there is not enough snow to present an avalanche hazard.Â
Observations
North
Observations from NCMG throughout the week continue to support a stabilizing upper snowpack. Reports show the 12/15 layer/PWL down 16-24 inches and reactive to snowpack tests. They observed moderate wind transport along ridges on Friday.Â
Central
On Thursday, professionals on Dirtyface Peak observed the 12/15 layer/PWL interface reactive in PST and ECT tests at 5000 ft on an east aspect. The interface was 2.5 ft down. Wind transport was occurring near and above treeline and actively loading lee slopes. Recent storm instabilities were healing.Â
South
No recent observationsÂ
Avalanche Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1